Composition and crystal structure of ternary phases in the Ta-Ni-Al system

The study covers the composition and crystal structure of compounds produced by self-propagating high-temperature synthesis (SHS) from the 5Ta—2Ni—3Al (at.%) powder mixture followed by vacuum remelting at 3000 °C. The SHS product contains the following phases: TaNiAl (Laves τ₁-phase), NiAl, Ni₂Al₃, and Ta. Its microstructure features by the presence ofternary phases with the composi­tion Ta₈₅Ni₇Al₈, Ta₅₂Ni₂₀Al₂₈ and Ta₅₃Ni₂₅Al₂₂ according to elemental analysis. The X-ray diffraction pattern of the remelted material revealed reflections that do not belong to any of the known ternary phases in the Ta—Ni—Al system considered. Based on the homological approach, it was found that these reflections belong to three phases with the structural types W₆Fe₇ (R3m), Ti₂Ni (Fd3m) and Ta3Al (P4₂/mnm). It was possible to define them as reflections of three compounds — Ta_6,5Ni_6,5, Ti₂Ni and Ta_2,84Al_0,91 with unit cell parameters different from those for the same compounds with the retained structural type. The increase in the unit cell parameters for all the phases identified was noted as compared to the known binary intermetallics. It may be associated with the presence of Al atoms in the crystal lattice for the Ta_6.5Ni_6.5 phase and Al and Ta atoms in the phase with the structural type Ti₂Ni. X-ray diffraction analysis and crystal-chemical modeling made it possible to identify Ta_6.5Ni_6.5 and Ti₂Ni phases as Ta₆Ni₆Al and Ta₂Ni_0.5Al_0.5, to determine their structural type, composition and unit cell parameters. Full-profile analysis was conducted to specify the structure and composition and determine unit cell parameters of the phases and their quantitative ratio in the material. The material phase composition is 47 wt.% Ta₆Ni₆Al, 16 wt.% Ta₂Ni_0.5Al_0.5 and 37 wt.% Ta₃Al.

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